The general structure of grinding machines is well known. Typically, a grinding machine has a hopper into which the material to be ground is placed, a grinder portion, including a grinding head, a mounting ring, a bridge, and a collection tube. A feed screw is located within the grinding head to advance material in the hopper through the head. A knife assembly is mounted at the end of, and rotates with, the feed screw and, in combination with the orifice plate, serves to grind material that is advanced toward the orifice plate by the feed screw. Typically, the orifice plate includes collection passages that lead to a collection cavity defined by a collection cone, which supplies material to a discharge passage. An orifice plate guard is located downstream from the orifice plate and maintains the collection structure in place, and a mounting ring holds a guard against the orifice plate and mounts the intervening structures to the body of the grinding head.
When frozen material is to be ground in a conventional grinding machine, the feed screw rotates in an internal chamber of the hopper to shear the frozen material. The internal chamber is defined by a longitudinal wall spaced from the feed screw. The frozen material is thus translated by the feed screw against the longitudinal wall as the frozen material is moved toward the orifice plate. This can place an undesirable side load on the feed screw. In addition, because the longitudinal wall is relatively smooth, the frozen material slides along the wall as it is moved toward the orifice plate. Moreover, the spacing of the wall from the feed screw can result in chunks that are sheared from the frozen material undesirably bouncing around as the feed screw rotates.
Another drawback of a conventional grinding machine is the limited number of shearing surfaces that are available. More particularly, in a conventional grinding machine, the frozen material can be sheared either by the knife at the forward end of the feed screw or by the pressure flighting on the body of the feed screw as the frozen material is pressed against the longitudinal wall of the internal chamber. However, as the block is reduced and/or the chunks of the block are bouncing around, it is difficult to hold the reduced blocks between the feed screw and the internal chamber wall. As such, reduced blocks of material may be advanced by the feed screw that are larger than desired.
Another drawback of conventional hoppers is the lack of post-reduction but pre-discharge volume. More particularly, a frozen block placed into the hopper will occupy a given volume. As the frozen block is sheared and thus reduced, the collective volume for all the reduced portions of the block will be greater than the volume originally occupied by the whole block. This is a result of air pockets that form between the sheared portions.
As noted above, conventional grinding machines use a knife positioned at a forward end of the feed screw. The knife is positioned in a knife holder that is coupled to the feed screw. The knife is an effective shearing tool as long as it is capable of withstanding the torsional loads placed on the knife during the shearing or grinding process.
Therefore, in accordance with one aspect of the invention, the internal chamber of a grinding machine includes one or more shearing edges that provide fulcrum points against which frozen blocks of material can be held to assist with shearing of the frozen blocks by a feed screw. The shearing edges may be arranged to limit the advancement of over-sized blocks by the feed screw.
In accordance with another aspect, the invention provides a grinding machine having a transition or expansion zone into which frozen material may be fed by the feed screw before ultimately being discharged by further advancement of the feed screw. The transition zone is designed to accommodate the increased volume of material that results as a frozen block is reduced.
In accordance with a further aspect of the invention, a feed screw for use with a grinding machine includes fins designed to provide support for a knife as the feed screw is rotated and the knife shears frozen material against the orifice plate.
It is therefore an object of the invention to provide a grinding machine that provides improved shearing efficiency.
It is another object of the invention to provide a grinding machine that provides improved control of the blocks as the blocks are moved toward the discharge of the grinding machine.
It is a further object of the invention to provide a knife holder that provides improved support for the torsional loads placed on a shearing knife used to shear frozen material.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description taken together with the drawings, which together disclose the best mode presently contemplated of carrying out the invention.